1. Field of the Invention
The present invention relates to components having multipactor-inhibiting coatings for use in any circumstances where the phenomenon of multipactor flashover occurs. For example, such components may be provided to high power RF energy devices, such as plasma sources, microwave devices, particle accelerating devices that generate high power, low frequency energy corresponding to radio frequency, microwave, or millimeter wavelengths, and that require an isolated vacuum environment in order to operate. These components may be components of klystrons, Inductive Output Tubes (IOTs), linear accelerators, magnetrons, RF amplifiers, UHF television transmitters, Traveling Wave Tube Amplifiers (TWTAs or TWTs), particle accelerators, triodes, radiotherapy devices, Free Electron Lasers (FELs), Backward Wave Oscillators (BWOs), ion accelerators, plasma devices, relativistic diffraction generators, RF transmitters, satellite systems, RF communication equipment, or other high power energy devices that generate or propagate energy corresponding to radio frequency, microwave, or millimeter wavelengths. These coated components may also be used in circumstances where the phenomenon of multipactor flashover does not occur but where it is desirable to selectively transmit electromagnetic energy at some frequencies while blocking or inhibiting transmission of other frequencies.
2. Description of Related Art
Multipactor flashover is a phenomenon that occurs in high power RF energy devices, wherein secondary electron emission in resonance with an electric field leads to exponential secondary electron multiplication resulting in an avalanche of electrons that damages or destroys components of the high power RF energy devices. The cause of this phenomenon involves high second electron emission (SEE) coefficients of the surfaces of the components that are exposed to the high power RF energy in the vacuum environment.
Waveguides of high power RF energy devices are components that may suffer from multipactor flashover. Common materials used in these waveguides include copper, silver, gold, and ALODINE, and high power devices commonly utilize oxygen-free copper with silver or gold plating. These materials are well-suited for waveguide applications based on RF response, but these materials share a major drawback: an unacceptably high secondary electron emission coefficient resulting in multipactor flashover events at relatively low field strengths. Diplexer components in satellite RF devices are a bottleneck due to their high rates of multipactor failure and, therefore, decrease overall performance. Recent research to address this problem includes simulation methodologies, new component test procedures, surface geometry modifications, and new processing techniques for multipactor mitigation coatings and other materials.
RF windows are components that also may suffer from multipactor flashover. Alumina is conventionally used for these RF windows. However, as a standalone material, alumina has an unacceptably high SEE coefficient resulting in multipactor flashover events at relatively low powers. One conventional option for improving the SEE coefficient of alumina RF windows is to coat the RF window with titanium nitride TiN, which has a lower SEE coefficient than alumina. However, TiN coatings can become chemically unstable upon exposure to air and are, therefore, unsuitable for many applications, such as space applications. Moreover, even if chemical exposure of the TiN coating is prevented, the improvement resulting from the TiN coating is limited. For a thin TiN coating, the danger of a multipactor flashover exists and, although a thick TiN coating will inhibit multipactor flashover, it fails due to overheating as a result of RF absorption. Recent research to address those deficiencies has focused on modeling the multipactor effect, optimizing the structure of the alumina dielectric, improving the processing of the TiN coatings, and replacing TiN with alternative materials.
With respect to improving the processing of the TiN coatings, it has been observed that the processing parameters for TiN significantly affect the resulting properties, and so some work has studied process optimization of TiN film growth. It has been found that argon ion assistance, or an optimized N2/argon mix during deposition, can lead to improved performance of the TiN coating during operation.
Also, AQUADOG, a water-based colloidal graphite suspension that was patented in the 1970s has been used for coatings in applications such as cathode ray tubes, and there has been some research in terms of publications and patents relating to diamond-like carbon coatings for these applications. However, diamond-like carbon coatings have not shown sufficient improvement in RF window properties.
Graphene is currently a major topic of research for semiconductor and other electrical applications. In some ways, graphene has comparable electrical properties to carbon nanotubes. Yet, graphene lends itself to lithographic processing techniques in a manner that leads to a more natural integration with current wafer processing technology and practices. Thus, the majority of graphene research is currently focused either on its semiconductor potential (while on its own graphene is a conductor, a number of different methods can induce the formation of gaps into its band structure) or the unique physics of graphene systems. For wafer-scale graphene, the most significant current method of graphene production is high temperature decomposition of silicon carbide. In this method, a SiC wafer is placed in a vacuum chamber where it may be hydrogen etched to produce a high-quality surface. Then the SiC wafer is annealed, either in a vacuum or in a controlled atmosphere, until the silicon on the surface desorbs, leaving behind an excess of carbon which then crystallizes into graphene using the SiC substrate as a template.
SiC has two geometrically different basal plane faces; the (0001) and the (1001), known as the ‘Si’ and ‘C’ faces, respectively. These two faces behave very differently during graphitization (with the faces maintaining their differing behaviors for different polytypes). In general, graphitization on the Si-face requires higher temperatures (1200-1300° C.) and results in the self-limited growth of three to five monolayers of graphene, whereas growth on the C-face begins around 150° C. cooler and can achieve thicknesses of 10 nm or more. Also, C-face growth results in a notably rougher morphology than Si-face growth, and both surfaces fail to yield uniform graphene films under all vacuum annealing conditions.
It has been suggested that in a vacuum, the rate at which Si sublimates (leaving behind excess C) always exceeds the rate at which the carbon mobility allows the graphene film to incorporate it, resulting in non-uniform graphene. Several methods to flip this problematic inequality have been tested with success: graphitization in a near-atmosphere argon environment, where the argon assumptively limits Si desorption simply by physical reflection; and graphitization under an incident Si molecular beam, where the desorption rate is countered by a significant absorption, and graphitization in a silane/disilane environment. Graphitization in these environments requires higher temperatures, but results in improved graphene crystallinity with larger domains In addition to graphitization of SiC, research has been conducted on coating graphene onto metals as well as on depositing graphene from solution for incorporation into large-area electronics.